1. Field of the Invention
This invention relates to noise limiters, particularly radio receiver noise limiters for limiting passage of impulse noise in the absence of an audio signal.
2. Background of the Invention
The typical noise limiter as heretofore used in radio receivers, and particularly aircraft radio receivers, contains a substantial disadvantage in terms of the amount of clipping which is possible. In such prior art limiters, the limiter is turned on with the carrier (or with the audio signal), and clipped when the signal exceeds a predetermined value. Such limiters inherently contain a trade-off between the two basic requirements of a noise limiter, i.e., the need to clip noise (and particularly impulse noise), and to pass the audio undistorted. The more effective the limiter is in clipping the impulse noise, the more serious is the distortion of the audio signal. Generally, the procedure followed is to specify the percentage modulation which is to be passed undistorted, thus providing a clipping level, and to clip both audio signal and noise which exceeds such level. In most modern solid state receivers the receiver white noise present at the input to the noise limiter does not exceed the clipping level and impulse noise which appears on top of the white noise is carried through to the limiter output, clipped only to the predetermined carrier modulation level. Thus, in operation of prior art noise limiter circuits, in the absence of an audio signal the impulse noise output from the limiter is substantially as great as the undistorted signal when audio is present.